I got the same results when I tested your script in CS5.5; it was okay when run via Script Debugger, but returned the raw code when saved as an app. I found the below in another discussion about coercing constants, and it seemed to work for me in CS5.5. Try adding this at the top of your script:
run script "tell application \"Adobe InDesign CC\"" & return & "end tell"
From what I understand, this may result in a bit of a performance hit as it's forcing the InDesign dictionary to load every time it's run, but it ran pretty quickly for me and did produce the correct dialog.
There is a problem with your code.
You're asking for measurement units before setting them.
If Indesign returns a constant expressed in <<chevrons>>, it's a bug.
But your script could be simplified:
tell application "Adobe InDesign CC"
set aspActiveDocument to active document
tell view preferences of aspActiveDocument
--get horizontal measurement units
set aspOrigHorizonalMeasurementUnits to horizontal measurement units
set aspOrigVerticalMeasurementUnits to vertical measurement units
--set horizontal measurement units
set horizontal measurement units to millimeters
set vertical measurement units to millimeters
set aspDocHeight to item 3 of bounds of page 1 of aspActiveDocument
set aspDocWidth to item 4 of bounds of page 1 of aspActiveDocument
set aspSizeInfo to (""& aspDocWidth & " x " & aspDocHeight & " millimeters") as string
display dialog aspSizeInfo